1. Field of Invention
This invention relates to a plant and process for washing sand using low energy techniques.
2. Description of the Prior Art
Conventional sand washing techniques known to the applicant comprise relative movement of sand and liquid on a continuous basis acting to remove clay and other impurities from the sand. The sand particles are classified as to size by floating off fine particles from the washing container. Several cycles of cleaning may be required to remove all of the impurities and consequently horsepower and water consumption is high to maintain an effective process in such conventional techniques.
Naturally occurring deposits of sands contain clay and ultra-fine silica particles and the aim in conventional plants is to remove the so called "fines" ranging in size up to 75 microns, the fines include silica and clay particles and may also include liquids if in fact the deposits relate to oil sands. The process retains the coarser sand particles.
It is normal practice to wash sand in a continuous process, and it is usual to introduce sand/gravel and a clay mixture at the top of a washing tank and allow it to descend against a rising body of liquid, which takes the clay and fine silica into suspension before it is floated off over a weir at the top of the tank. If the sand is not clean by the time it has reached the bottom of the tank, it is usual to repeat the performance by pumping the sand and liquid to the top of a second tank, and so on until the desired state of cleanliness is achieved. Two, three and sometimes four such cycles may be required.
Other washing methods are used, for instance in ore classifiers as disclosed in U.S. Pat. No. 1,984,362, large diameter screws are fitted to inclined troughs, where water or like liquid and sand are added. The screws are rotated thus causing the sand to travel up the incline and be washed by the liquid being stirred with it. If it is not cleaned in the first pass, the process is repeated. A similar unit is disclosed in U.S. Pat. No. 3,043,430. Another method is to place high pressure water/sand nozzles opposed to each other in a tank as disclosed in Australian Patent No. 564412. The colliding liquid/sand streams cause the removal of contaminants from the silica such as clay and contaminant liquids. All of these methods utilise relatively high quantities of power. Another version noted is a hydro-deslimer as disclosed in U.S. Pat. No. 3,070,228 which provides a counterflow of water in a trough in which water is injected through small holes in the bottom of the trough.
Other methods of separation are disclosed for example in U.S. Pat. No. 3,152,979 relating to the separation of oil from tar sands in which water and air are introduced in a trough to assist in the separation of oil. The use of air in the assistance in separating sand particles is known and for example has been disclosed in Soviet Union specification Nos. 1445789 and 1577835.
It has been noted that there is a problem of washing of sand with a mixture of air and liquid since both the air and the liquid tend to take the line of least resistance through the matrix or pulp of sand clay thereby making tracks but-leaving areas of sand virtually untouched or unwashed. Thus with existing methods of air/liquid washing of sands inefficiencies are apparent which the present invention sets out to address.